In recent years, public awareness of the need to conserve valuable resources and the lack of acceptable means for disposing of solid waste has led to the adoption of numerous voluntary and mandatory recycling measures.
Polymeric waste materials have received particular attention because of their resistance to biodegradation. Public pressure to reduce the volume of solid waste interred in landfills, coupled with the continuing strong demand by consumers for synthetic polymer products, has thus prompted a rapid expansion in the development of polymer recycling technology.
Strategies for secondary recycling of plastics can be divided into two major areas: (1) those emphasizing material separation and ultimate fabrication of similar end-use products; and (2) those devoted to reprocessing of co-mingled waste to replace concrete and wood in products which do not require the physical properties of the virgin materials. Naturally, co-mingled polymeric waste is often contaminated with other materials, such as wood, paper, metals (both ferrous and nonferrous) and glass. Removal of these non-polymeric components is necessary both to protect equipment from harmful abrasives and to achieve physical properties in the reprocessed material which are reasonably close to those of virgin resin. Furthermore, certain thermoplastic mixtures, such as polyvinylchloride ("PVC") and polyethylene terphthalate ("PET"), can lead to material and equipment degradation when reprocessed together. Finally, experience has shown that physical properties, and thus resale value, of recycled polymeric materials, increases as the purity of the material increases. Consequently, schemes capable of selectively separating each polymeric component from a co-mingled mixture enhance the value of waste thermoplastics.
Synthetic polymer waste streams are generally composed primarily of high and low density polyethylene ("HDPE" and "LDPE"), polypropylene ("PP"), polystyrene ("PS"), both in foamed and bulk form, PET and PVC. The recycling of these thermoplastics has been limited by difficulties in separating the polymers from each other and from non-polymeric contamination. Current schemes to separate thermoplastic waste generally rely on a combination of hand-sorting and either hydrocloning or air classification. A primary drawback in the use of the latter techniques is that several commercially vital separations, such as PVC from PET, or classification of the olefin component of the waste stream, are not feasible using these technologies. Furthermore, component selectivity in both air classifiers and hydroclones is a function of particle size distribution, as well as particle density, which limits overall separation efficiency. The separation efficiencies of these techniques are, therefore, relatively poor, and it is necessary to resort to hand sorting. The disadvantages intrinsic to hand-sorting are obvious.
Although optical-mechanical processes will remove problematic materials such as PVC from the waste stream, such processes may prove suitable only to handling fully-intact thermoplastic bottles. A large portion of the plastic waste stream, however, is likely to arrive at a re-processor in a shredded and bailed form.
Waste paper products have also received particular attention, primarily because of the enormous volume involved. The enormous volume of waste paper interred in landfills has greatly contributed to the current scarcity of available landfill space. The need to recycle paper products is thus of equal if not greater magnitude than the need to recycle plastics.
Current technology allows only for the reprocessing of newsprint for the ultimate fabrication of a similar end-use product. Other grades of paper can be reprocessed for use in a variety of products not requiring the physical properties of the virgin materials. Glossy paper and other paper grades are currently separated by hand prior to de-inking and reuse of the newsprint.
A definite need for an effective method for selectively separating co-mingled materials of different densities therefore exists.